The present invention relates to a glass waveguide for optical circuits and a method for fabricating such a waveguide.
In fabricating a glass waveguide for optical circuits, a core layer composed of a glass film is formed on a glass substrate by a sputtering process. The refractive index of the glass film is higher than that of the glass substrate. The core layer having a high refractive index is then etched into a desired pattern by a photolithographic method or the like. Finally, the pattern is coated by material having a lower refractive index. A light beam propagates through the waveguide thus formed, and is mainly concentrated in the core having the higher refractive index. Various methods have been proposed for fabricating this type of glass waveguide. The waveguide fabricated by any of these methods is a thin film having a thickness of approximately 1 .mu.m or less, because of restrictions in the waveguide fabrication method. It has been desired to form a core having a thickness of 5 to 50 .mu.m, in order to increase the efficiency with which connections can be made to optical fibers or other optical circuit elements and to realize a low loss waveguide. In addition, it is preferable that the core be coated with an upper cladding layer having a lower refractive index. Furthermore, it is necessary that the cross sectional configurations and dimensions of the waveguide be controlled in a precise manner in order to realize various functions thereof as an optical circuit element. It is not necessarily easy to fulfill all of these requirements. A particularly serious problem in realizing integrated optical circuit is that the core is deformed while forming the upper cladding layer following the formation of a core having a desired pattern.
When the core layer has a thickness of 1 .mu.m or less, a difference between the expansion coefficients of the substrate material and the thin film material is out of the question. In the case of a glass waveguide for an optical circuit having a thick film glass in which the thickness of the core is 5 to 50 .mu.m, a metal oxide such as GeO.sub.2, P.sub.2 O.sub.5, TiO.sub.2 or the like is added to SiO.sub.2 for the purpose of controlling the refractive index of the glass. The addition of the metal oxide changes the expansion coefficient of the glass. As a result, there is the possibility that the expansion coefficients of substrate glass made of, for example, silica (SiO.sub.2) and a glass layer forming the waveguide will often change greatly. If the expansion coefficients differ between the substrate glass and the glass layer thereon, a distortion is produced due to a temperature change in the fabricating process of the waveguide. In an extreme case, the glass layer may be broken down. Even if the glass layer is not broken down in the fabricating stage, a slight weight applied to a waveguide formed under such conditions may damage the waveguide while it is in use. Thus, the waveguide is unstable.
Because of these problems, no practically usable waveguides for optical circuits have yet been developed. Optical fibers, prisms, lenses, mirrors and the like are still used as components for the formation of optical circuits.
U.S. Pat. No. 3,806,223 by Donald B. Keck et al., entitled "Planar Optical Waveguide" and U.S. Pat. No. 3,934,061 by the same inventors, entitled "Method of Forming Planar Optical Waveguides", disclose how to fabricate waveguides of relatively high quality. A waveguide manufactured by the method disclosed in these Keck et al patents, however, still involves a disadvantage in that the dimensions of the waveguide core, the configuration of the core cross section and the expansion coefficient difference are not controlled precisely. More specifically, in the Keck et al method, fine glass particles are formed by flame-hydrolyzing raw material, such as SiCl.sub.4 or GeCl.sub.4. Flame hydrolyzing is carried out by means of a reaction burner, for example, an oxy-hydrogen burner, and the fine glass particles are deposited on a glass substrate. Then, a portion of the fine glass particles located where a waveguide core is to be formed is vitrified into a core of transparent glass by irradiating the fine glass particles with a CO.sub.2 gas laser. In the next step, the non-irradiated portion of the fine glass particles is removed. Subsequently, a second portion of fine glass particles is deposited on the core and the substrate, and then is vitrified into a transparent glass to form a cladding layer. In the core formation step, the reactive burner may be moved along a desired pattern to deposite fine glass particles locally, which is then vitrified into a transparent glass, as disclosed in U.S. Pat. No. 3,806,223.
In these methods, the fine glass particles, flame-hydrolyzed by moving the substrate relative to the burner, are progressively deposited. Therefore, fluctuations of the flame and a minute change in the flow the rate of oxy-hydrogen gas cause a change of thickness and composition of the fine glass particles deposited on the substrate. This makes it difficult to form an optical waveguide having uniform characteristics. There has also been proposed the vitrification by the laser or the local deposition of the core layer for the purpose of patterning the core layer.
It is, however, difficult to precisely control dimensions such as the height and width of the waveguide, which are important parameters of the waveguide. Particularly, it is impossible to fabricate a waveguide of which the width and height must be 5 to 10 .mu.m, such as a single mode waveguide. The indefinite shape of the core cross section raises a serious problem from the viewpoint of making a connection between waveguides or between a waveguide and an optical fiber.
In order to improve the Keck et al method, the present inventors have previously proposed in Japanese patent application No. 75036/1978 (document 55-02263, dated Jan. 9, 1980) a new method of forming a glass layer, in which the refractive index and the thickness are carefully controlled, on a substrate. In this method, in order to form a uniform glass layer, a substrate placed in a reaction vessel is maintained at a high temperature of 1200.degree. C. to 1650.degree. C. Under this condition, glass raw material such as SiCl.sub.4 or GeCl.sub.4, together with oxygen, is introduced into the vessel where the glass raw material is thermal-oxidized into glass oxide to form a transparent glass layer on the substrate. This method enables a uniform glass layer to be formed over the entire surface of the substrate. In this method, however, the optimum temperature of the oxidation reaction of the glass raw material is different from the optimum temperature at which the synthesized glass is deposited on the substrate in a transparent state. Accordingly, the temperature range which satisfies both of the requirements is narrow. If the temperature deviates from the optimum temperature range, the refractive index and the thickness of the glass layer change. In this respect, it is not necessarily easy to form a uniform glass layer with good reproducibility. In addition, it is extremely difficult using this method to form optical waveguides having a high refractive index difference by adding additives having a high vapor pressure at a high temperature such as GeO.sub.2 or P.sub.2 O.sub.5. Accordingly, it is not possible to form a glass layer having a refractive index which is higher by 0.5% or more than that of pure silica glass.
In forming an optical waveguide, some suitable dopant(s) is(are) added to the core layer in order to control its refractive index. As a result, the softening temperature of the core layer is lowered, so that the core layer is deformed by heat application when an upper cladding layer is deposited onto the core layer. The deformation makes it difficult to control the configuration of the cross section and the dimensions of the core layer. Accordingly, it has been almost impossible to form a glass waveguide having characteristics, such as the propagation constant, which are within a given tolerance.
Usually, an optical waveguide is so designed that there is a refractive index difference of 0.2 and 3% between a core layer and its surrounding part. Because of this, the maximum expansion coefficient difference between the core layer and the silica glass substrate reaches 3.times.10.sup.-6. As a result, the glass layer for the core easily cracks and is apt to be broken. In this respect, it has been desired to fabricate a glass waveguide for an optical circuit which is stable with respect to temperature change.
In a conventional optical waveguide as mentioned above, an irregular boundary itself at the side of the core, which is produced in the fabricating stage of the core layer, serves as a boundary of the waveguide. Accordingly, there is the disadvantage that the scattering loss of the guided light is large, i.e., a waveguide with a low loss cannot be obtained.
There has also been proposed an embedded type glass waveguide in which ions for increasing the refractive index are diffused into glass, as disclosed in Japanese Patent Application Publication No. 5975/1973 (document 48-05975, dated Feb. 21, 1973). In this waveguide, a core layer is formed by diffusing ions, so that the boundary surface of the waveguide is not irregular and a waveguide with a low scattering loss is obtained. However, the waveguide formed by this proposal has the following drawbacks:
(1) This method employs a diffusion phenomenon, so that it is difficult to control precisely the dimensions of the waveguide. This makes it difficult to fabricate a single mode waveguide requiring a core dimension of about 10 .mu.m. PA0 (2) It is difficult to obtain a fixed cross sectional configuration of the waveguide and thereform to form a cross section with a desired shape such as a circle or rectangle.